How do I export material preview scene data so I can render it with my own engine?
Two options:


If you really wanted to go the route of exporting the material preview scene, which would allow for forward compatibility in case the preview scene should ever change in blender, the only option would probably be to create a temporary scene, copy all the data from the scene received in the render_preview function and then export that. 


But this is probably not worth the hassle, so I recommend to anybody who needs something like this:


Get the material preview scene as a .blend from the blender code repository and export that once. Finally in the render_preview method, just manually serialize the material properties to get those into your custom engine.


I hope this helps somebody else struggling with this.